A known example of a motor including an armature and a field element which are rotatable relative to each other on a rotation axis is an axial gap brushless DC motor (which will hereinafter be simply referred to as a “brushless motor”). In general, in an axial gap brushless motor, a rotor includes a magnetic-field creating magnet and a stator includes an armature winding and a yoke plate. Then, a structure in which the magnetic-field creating magnet and the armature winding face each other in an axial direction parallel to a rotation axis is employed.
For the magnetic-field creating magnet of the rotor, a magnet which is plate-shaped and has magnetic poles which are arranged along a thickness thereof (orthogonal to a flat portion) and are different from each other in polarity is employed, for example. Then, the magnetic-field creating magnet is placed such that the flat portion thereof is orthogonal to the rotation axis in order to allow a direction of a magnetic flux of the magnetic-field creating magnet to be parallel to the rotation axis. Also, the armature winding and the yoke plate of the stator are stacked along the rotation axis in order of mention when viewed from a side on which the rotor is placed. The yoke plate is formed of a magnetic plate and is placed such that a flat portion thereof is orthogonal to the rotation axis.
To flow a predetermined current through the armature winding of the foregoing brushless motor causes the magnetic-field creating magnet to function as a magnetic field, so that the rotor makes rotational movement relative to the stator.
For a motor which is characterized in a flat shape thereof such as the foregoing axial gap brushless motor, axial thinning is demanded. However, when a magnet for creating a field magnet and an armature winding face each other in an axial direction as described above, respective widths of the magnetic-field creating magnet and the armature winding, which widths extend in the axial direction, are added to each other, to have difficulties in thinning the motor. To overcome those difficulties, a structure in which a magnetic-field creating magnet and an armature winding do not face each other in an axial direction but face each other in a radial direction perpendicular to the axial direction has been suggested in recent years (refer to Patent Documents 1, 2, and 3, for example).
[Patent Document 1] Japanese Patent Application Laid-Open No. 5-344701
[Patent Document 2] Japanese Patent Application Laid-Open No. 59-216458
[Patent Document 3] Japanese Patent Application Laid-Open No. 59-516459
[Patent Document 4] Japanese Patent Application Laid-Open No. 8-124736
[Patent Document 5] Japanese Patent Application Laid-Open No. 6-46554
[Non-patent Document 1] Takaharu Takeshita and other two, “Control for Sensorless Brushless DC Motor Based on Error in Current Estimation”, IEE Transactions D, volume 115, No. 4, pp. 420-427, 1995.
[Non-patent Document 2] Takaharu Takeshita and other three, “Control for Sensorless Brushless DC Motor with Salient Pole Based on Estimation of Motional Electromotive Force”, IEE Transactions D, volume 117, No. 1, pp. 98-104, 1997.